[BLDG-SIM] Citi Multi Variable Refrigerant System

[Dana Troy]

Hello all,
 
We have recently performed an analysis on a building that is potentially going to incorporate the City Multi system. After many hours of going through eQUEST, I devised a method of modeling it that I would like to run past you all for any thoughts/ideas. 
 
First, for those not aware of the problems, here is a background of the systems:
 
The City Multi system consists of essentially three parts: the condenser, the BC controller, and the heat exchanger unit at each zone. Refrigerant is the only property used for heat exchange, i.e. there is no water running from the condenser to the controller to the zone units. Each zone unit is essentially a fan coil, in that there is no compressor work at the zone, only fan energy. The BC controller is essentially a heat exchanger which gives the City Multi system a large advantage by being able to transfer heat, say from the south face of a building, to the north face, which is in the shade. All of the individual zone units connect to the BC controller and the BC controller distributes the refrigerant accordingly. If there is not enough free energy being generated by the system, the compressor kicks in and supplies or rejects heat from the loop to meet the demand. 
 
To model this in eQUEST, which has been suggested before, the water source heat pump system should be selected in order to account for the free transfer of heat from zone to zone. There are, however, several problems with this. One is that pumping energy is required to pump the water throughout the building. The second problem is that at each zone, each heat pump runs a compression cycle in order to supply or reject heat to or from the zone to the water loop. The third is that a boiler and cooling tower take the place for the condenser in the City Multi system. Each system requires a make-up air unit which can be modeled the same in each simulation.
 
Now here is what I would like you all to review: 
 
These are some of the steps I went to in order to make the water source heat pump as much like the City Multi as possible.

*	Made the boiler and electric boiler (to simulate the electric compressor) and gave it the appropriate heating EIR that was specified in the Citi Multi specification manual. I gave the fan the same EIR as the cooling side of the compressor in the same manual. 

*	I made a spreadsheet to subtract out the pumping energy (only from the water loop! Not the DHW loop!) to simulate the suction of the refrigerant through the building in the City Multi system. I had to do this in the BEPS and the BEPU report which made it kind of a hassle to find the energy cost savings. 

*	For each heat pump, I set the EIR equal to 0.0001 to simulate the fan coil in the City Multi system. Therefore, whatever heat is transferred to or from the water loop is only shown with the fan energy from blowing the air over the coils. IF this is not a proper way to do it, then what should the EIR be? It cannot be the EIR from the condenser, because that has nothing to do with the free heat transfer energy at the zones. 

What do you guys think about this approach? I figure that since this is a tough problem that other people have encountered as well, I might as well use this huge resource that has many great minds to come to a solution. 
 
Please, any feedback would be greatly appreciated. 
 
 
Regards, 

Dana Troy
Energy Analyst 

 <http://www.glumac.com/>  
  _____  

320 SW Washington, Suite 200
Portland, OR 97204-2640
T.  503.227.5280  F. 503.274.7674
D. 503.345.6286

Thinking. Inside the building.
www.glumac.com <http://www.glumac.com/>  
 


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